Polarization rotator and a crystalline-quartz plate for use in an optical imaging system

ABSTRACT

A polarization rotator and crystalline quartz plate for use with an optical imaging system. The system has several imaging optical components (L 1 –L 16 ) sequentially arranged along an optical axis ( 16 ), a means for creating radially polarized light arranged at a given location in that region extending up to the last of said imaging optical components, and a crystalline-quartz plate employable in such a system. A polarization rotator ( 14 ) for rotating the planes of polarization of radially polarized light and transforming same into tangentially polarized light, particularly in the form of a crystalline-quartz plate as noted above, is provided at a given location within a region commencing where those imaging optical components that follow said means for creating radially polarized light in the optical train are arranged. The optical imaging system is particularly advantageous when embodied as a microlithographic projection exposure system.

This is a continuation of application Ser. No. 10/883,849 filed Jul. 6,2004, which in turn is a divisional application of prior applicationSer. No. 10/145,138 filed May 15, 2002 (now U.S. Pat. No. 6,774,984).The entire disclosures of the prior applications, application Ser. Nos.10/883,849 and 10/145,138, are considered part of the disclosure of thepresent continuation application and are hereby incorporated byreference. The following disclosure is based on German PatentApplication No. 101 24 566.1, filed on May 15, 2001, which is alsoincorporated in this application by reference.

FIELD OF AND BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a polarization rotator and crystalline quartzplate for use with an optical imaging system having several imagingoptical components arranged in succession along an optical axis, meansfor creating radially polarized light arranged within that regionextending up to the last of said imaging optical components, and acrystalline-quartz plate employable on such a system.

2. Description of the Related Art

German laid-open publication DE 195 35 392 A1 discloses an opticalimaging system of said type in the form of a microlithographicprojection exposure system having, e.g., an i-line mercury dischargelamp as a light source. Said system's employment of radially polarizedlight for exposing wafers was intended to improve the coupling of saidlight into the layer of photoresist, particularly at very large anglesof incidence, while simultaneously achieving maximum suppression of anystanding waves that might be caused by reflections at the inner andouter interfaces of said photoresist. Various types of radial polarizersthat employ birefringent materials were mentioned as prospective meansfor creating radially polarized light. That radial polarizer chosen wasarranged within that region that followed said system's finalphase-correcting or polarizing optical element in the optical train inorder that the degree of radial polarization attained prior to incidenceon said wafers would remain unchanged. In the event that a catadiopticoptical system were employed as said system's projection lens, theradial polarizer involved should be preferably arranged, e.g., followingsaid optical system's final deflecting mirror. Otherwise, it might bearranged, e.g., within the preceding illumination system of theprojection exposure system.

Radially polarized light, i.e., light that is linearly polarizedparallel to its plane of incidence on an interface, is, in general,preferable in cases involving imaging optics, e.g., the imaging opticsof microlithographic projection exposure systems, since radiallypolarized light allows employing highly effective antireflectioncoatings on their imaging optical components, particularly their lenses,which is a matter of major importance, particularly in the case ofmicrolithographic projection exposure systems with high numericalapertures and at short wavelengths, e.g., wavelengths falling in the UVspectral range, since there are few coating materials that are suitablefor use in that spectral range. On the other hand, tangentiallypolarized light, i.e., light that is linearly polarized orthogonal tothe plane of incidence of an imaging light beam on the respectiveinterfaces of the lenses, or similar, involved, should preferably beemployed for illumination in order to allow creating the best possibleinterference-fringe contrasts when imaging objects on, e.g., wafers. Inorder to allow same, the older German patent application 100 10 131.3proposed employing a tangentially polarizing element arranged in thevicinity of a pupillary plane of the projection lens, or within theillumination system that precedes same in the optical train that may beassembled from segmented birefringent plates instead of the radialpolarizer of German patent disclosure DE 195 35 392 A1.

OBJECTS OF THE INVENTION

The invention is based on the technical problem of providing apolarization rotator and a crystalline quartz plate for use in anoptical imaging system of the type mentioned at the outset that willboth allow comparatively highly antireflective coatings on its optics,which will minimize disturbing reflected stray light, and be capable ofyielding an exiting light beam that will allow creating high-contrastinterference fringes on an image plane.

SUMMARY OF THE INVENTION

According to one formulation, the invention solves these and otherobjects by providing a polarization rotator and a crystalline quartzplate for use in an optical imaging system, in particular amicrolithographic projection exposure system, that includes severalimaging optical elements arranged one after the other along an opticalaxis, and a radial polarizer radially polarizing light transiting saidoptical imaging system and arranged at a location ahead of the finalimaging optical element. The polarization rotator transforms theradially polarized light into tangentially polarized light and isarranged at a location following that imaging optical element thatfollows said radial polarizer in the optical train. The inventionadditionally addresses these objects by providing a crystalline-quartzplate configured as a polarization rotator, wherein a crystal axis ofsaid plate is at least approximately normal to the plane of said plate.

The optical imaging system according to the invention is characterizedtherein that it both provides a means for creating radially polarizedlight with which at least part of the imaging optical components of saidsystem operate, and provides a polarization rotator for rotating theplanes of polarization of said radially polarized light and fortransforming same into tangentially polarized light in order to yieldlight that will be tangentially polarized in an imaging plane. Saidpolarization rotator is arranged following at least one, and preferablyseveral, or even all, of the imaging optical components of said system.

A consequence of said measures according to the invention is that allimaging optical components of said system that are situated between saidmeans for creating radially polarized light and said polarizationrotator may operate with radially polarized light, for which they may behighly effectively antireflection coated. In particular, a conventionaltype of radial polarizer situated at an arbitrary location in the beampath between said light source, i.e., a location ahead of said system'sfirst imaging optical component, and said system's final imaging opticalcomponent, but ahead of said polarization rotator, may serve as saidmeans for creating radially polarized light. Said polarization rotatorwill simultaneously transform said radially polarized light, which ispreferable for the imaging optical components involved, intotangentially polarized light that will then be incident on said imageplane, which will allow creating high-contrast interference fringesthereon. Since said polarization transformation is effected by rotatingplanes of polarization, the associated intensity losses may be held tolow levels.

Under another embodiment of the invention, a plate having an opticallyactive material is employed as said polarization rotator. Opticallyactive materials are known to rotate the planes of polarization oftransmitted light, where the angles through which same are rotated willbe proportional to the thicknesses of said materials and the constantsof proportionality involved will increase as the wavelengths involveddecrease. Under another embodiment of the invention, acrystalline-quartz plate serves as said polarization rotator. Althoughsaid crystalline-quartz plate will also have birefringent properties,suitably dimensioning and orienting said plate will allow maintainingsame at levels so low that the desired polarization rotation will not besignificantly altered by the optical activity of said crystallinequartz, at least not in cases involving UV-light, e.g., light havingwavelengths of about 157 nm or less.

Under beneficial other embodiments of the invention in which saidoptical imaging system is a microlithographic projection exposuresystem, said polarization rotator for rotating the planes ofpolarization of radially polarized light and transforming same intotangentially polarized light is arranged within a section of saidsystem's projection lens where the beam path is approximately parallelto its optical axis, in particular, in a pupillary plane, or within asection lying between a pupillary plane and an image plane of samecontaining, e.g., a wafer to be illuminated. In the case of the first ofsaid arrangements, arranging said polarization rotator in said pupillaryplane has the advantage that the approximately normal incidence of lighton said polarization rotator yields a high optical activity of same andthe effects of off-axis illumination of same, such as birefringenceeffects, will remain minimal. On the other hand, arranging saidpolarization rotator closer to said image plane has the advantages thatthose imaging optical elements situated between said pupillary plane andsaid polarization rotator will also be penetrated by radially polarizedlight and that employment of a smaller polarization rotator will besufficient.

In the case of the crystalline-quartz plate according to the invention,the crystal axis of said plate is oriented approximately parallel to thenormal to its surface. A crystalline-quartz plate having saidorientation is particularly well-suited to employment as a polarizationrotator on optical imaging systems according to the invention.

Under a modified embodiment of the invention, the thickness of saidcrystalline-quartz plate is 500 μm or less and preferably about 200 μmor less. Plates that thin are particularly suitable for accomplishingthe polarization-rotation function on optical imaging systems accordingto the invention when operated at far-UV wavelengths of 157 nm or less.

BRIEF DESCRIPTION OF THE DRAWINGS

A beneficial embodiment of the invention is depicted in the accompanyingdrawings and will be described below. Said drawings depict:

FIG. 1: a schematic drawing of a microlithographic projection exposuresystem that has a means for creating radially polarized light arrangedwithin its illumination system and a polarization rotator for rotatingthe planes of polarization of same and for transforming same intotangentially polarized light arranged within its projection lens and

FIG. 2: a detailed drawing of the projection lens depicted in FIG. 1.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 depicts a conventional microlithographic projection exposuresystem that is similar to that cited in German patent disclosure DE 19535 392 A1, except for the arrangement of its polarization rotator withinits projections lens. Light from a light source (1), e.g., an i-linemercury discharge lamp, that emits illuminating UV-radiation at adesired wavelength focused by a mirror (2) illuminates an aperture stop(3) that is followed by a lens (4) such as, in particular, a zoom lens,and that allows making various adjustments, in particular, choosing adesired circular aperture. Instead of a mercury discharge lamp, a laserlight source emitting at a wavelength of around 260 nm or less, e.g., at157 nm, may be employed as said light source (1), in which case saidmirror (2) will be superfluous.

A radial polarizer (5) that transforms unpolarized incident light intoradially polarized light is arranged following said lens (4). Saidradial polarizer (5) may be, e.g., a truncated-cone polarizer having theconfiguration described in German patent laid-open publication DE 195 35392 A1, which performs said transformation without causing significantlight losses. The resultant largely radially polarized light thentravels from said radial polarizer (5) to a honeycomb condenser (6) anda relay and field lens (7) that follows same in the optical train. Thelatter components collectively serve to provide optimal illumination ofa mask (8), which is also termed a “reticle,” bearing the pattern to beimaged. A projection lens (9), which has been configured as a reducinglens and follows said components in the optical train, images saidpattern, which lies in the object plane of said projection lens (9),onto a film of photoresist (10) on a wafer (11) lying in the image planeof said projection lens (9) with ultrahigh spatial resolution,preferably with a spatial resolution of better than 1 μm. The numericalaperture of said system should preferably exceed 0.5, in particular,should preferably range from 0.7 to 0.9.

FIG. 2 schematically depicts a prospective configuration of saidprojection lens (9), which has numerous lenses (L1–L16). Since many ofthe lens arrangements typically employed on projection lenses of thattype are known, those lenses (L1–L16) that have been depicted in FIG. 2are to be interpreted as representing lenses typically employed on saidconventional types of lens arrangements and have thus been symbolicallyindicated by rectangles that, of course, are not intended to representtheir true geometric shapes. In order to clarify the operation of saidprojection lens (9), the paths of the principal rays (12 a, 13 a) andthe marginal rays (12 b, 13 b) of the imaging beams (12, 13) associatedwith a central point (8 a) of said mask and a point (8 b) near the edgeof said mask, respectively, have been schematically indicated.

The distinctive feature of the projection lens depicted in FIG. 2 is itsarrangement of a polarization rotator (14) that, in the case of thisparticular example, is situated right after a pupillary plane (15) ofsaid projection lens where a typical aperture stop is arranged. Saidpolarization rotator (14) has been designed to rotate the planes ofpolarization of incident radially polarized light and transform sameinto tangentially polarized light. A thin crystalline-quartz plate whosecrystal axis (17), which has been schematically indicated in FIG. 2, isoriented approximately parallel to the optical axis of said projectionlens may be employed in an exemplary embodiment of said polarizationrotator, where said crystal axis (17) of said crystalline-quartz plateis oriented approximately orthogonal to the plane of said plate, i.e.,approximately parallel to the normal to its surface.

Crystalline quartz is known to be optically active and, unlike the caseof normal birefringence, rotates the planes of polarization of incidentlight, regardless of their original orientations, due to its opticalactivity. Another advantage of optically active materials is that theycreate no double images. The angle of rotation for a given material willbe proportional to its thickness, where the constant of proportionalityinvolved will vary with its temperature and be largely determined by thewavelength involved. In the case of the application involved here, it isparticularly beneficial that said constant of proportionality markedlyincreases with decreasing wavelength and is several times greater forwavelengths falling within the UV spectral range, e.g., the wavelengthrange 150 nm to 260 nm, than for visible light. This is the reason whyit will be sufficient to employ a very thin crystalline-quartz platewhose thickness is only around 500 μm, and preferably 200 μm or less, inorder to produce the desired rotation in cases where UV-radiation isemployed on microlithographic projection illumination systems. Sincebirefringence effects will not simultaneously significantly increase atshorter wavelengths, the ratio of the aforesaid desired function of saidoptical activity to any disturbing birefringence effects will becorrespondingly improved at short wavelengths falling within the UVspectral range.

Arranging said polarization rotator (14) near said pupillary plane (15)or at some other location in the beam path where light rays propagateparallel to, or at a small angle of inclination with respect to, saidoptical axis (16) has the advantage that light rays incident on samewill be approximately normal to its surface, in which case the ratio ofsaid desirable function of said optical activity to said, in the case ofthe example considered here, undesirable, birefringence effects ofcrystalline quartz, will be particularly large. In the case of thatparticular location of said polarization rotator (14) shown in FIG. 2,eleven of said sixteen lenses (L1–L16) of said projection lens and theentire optical train of said illumination system, commencing with saidradial polarizer (5), will lie within that portion of the beam pathwhere light is largely radially polarized. This will allow providinghighly effective antireflection coatings on the lenses involved, whilesaid polarization rotator (14) will provide light incident on said wafer(11) that has the desired, largely tangential, polarization.

Alternatively, said polarization rotator (14) may also be positioned atany arbitrary, location along said optical axis (16) of said system, butshould preferably be positioned as close as possible to said image planeor said wafer (11) in order to ensure that as many as possible of saidimaging optical components will be penetrated by radially polarizedlight. Relocating said polarization rotator (14) from the vicinity ofsaid pupillary plane (15) to a location closer to said wafer (11) willallow choosing a smaller diameter for said polarization rotator (14),while providing that at least some of those lenses (L12–L16) situatedbetween the indicated location of said polarization rotator (14) andsaid wafer (11) will still be irradiated by radially polarized light.However, the divergence, i.e., the maximum angle of inclination withrespect to said optical axis (16), of the beam incident on saidpolarization rotator (14) will then increase.

The ratio of the strength of said optical activity to that of saidbirefringence effects will decrease with increasing angle of incidence,which will slightly worsen the effects due to said crystalline-quartzmaterial's birefringence. However, decisions regarding the maximumangles of incidence that may be tolerated may be made based on theparticular applications to be involved. Those decisions will also dependupon the extent to which light has been radially polarized with respectto the optical axis of the crystal of said polarization rotator (14)prior to its arrival at same, since, in the ideal case of totallyradially polarized light, no birefringence effects will occur, even forhigh beam divergences, i.e., at large angles of incidence on same.However, said ideal case will usually be unachievable in actualpractice, since light supplied by said illumination system will not beperfectly radially polarized and slight departures from perfect radialpolarization will occur due to stress-induced birefringence in saidlenses. Nevertheless, fairly high beam divergences may be tolerated dueto the resultant very high degrees of optical activity, particularly atshort UV-wavelengths, and said polarization rotator (14) might even bepositioned between the last of said lenses (L16) and said wafer (11).The latter placement of said polarization rotator (14) has theparticularly beneficial advantage that all imaging optical components ofsaid optical imaging system will be able to operate with radiallypolarized light and said polarization rotator (14) will no longer needto be incorporated into said projection lens, i.e., may be positionedoutside same.

The foregoing description of a beneficial sample embodiment makes itclear that an optical imaging system according to the invention willallow achieving high-quality imaging largely free of the disturbingeffects of stray light by providing that a large majority of saidimaging optical components, preferably at least ⅔ thereof, will beirradiated by radially polarized light for which said imaging opticalcomponents have highly effective antireflection coatings. Said opticalimaging system will also be capable of providing a largely tangentiallypolarized beam that will allow creating high-contrast interferencefringes, such as those that will be of benefit when same is employed as,e.g., a microlithographic projection illumination system for exposingphotoresists on wafers, at its image plane.

1. A microlithographic projection exposure system, comprising: aplurality of imaging optical elements arranged as an optical train,wherein the plurality of imaging optical elements is arranged along anoptical beam path and includes a projection lens; and a crystallinequartz plate which rotates a polarization of an entire imaging beampassing through the projection lens by the optical activity of thecrystalline quartz plate.
 2. The microlithographic projection exposuresystem according to claim 1, wherein an optical axis of the crystallinequartz plate is aligned parallel to beam axis defined by the imagingbeam passing through the crystalline quartz plate.
 3. Themicrolithographic projection exposure system according to claim 1,wherein the plurality of imaging optical elements is arranged along acommon optical axis, and wherein the crystalline quartz plate isdisposed within the projection lens at a location where a path of theimaging beam is substantially parallel to the optical axis of theplurality of imaging optical elements.
 4. The microlithographicprojection exposure system according to claim 1, wherein the crystallinequartz plate is disposed between a pupillary plane and an image planewithin the projection lens.
 5. The microlithographic projection exposuresystem according to claim 1, wherein the plurality of imaging opticalelements is arranged along a common optical axis, and wherein an opticalaxis of the crystalline quartz plate is aligned parallel to the opticalaxis of the plurality of imaging optical elements.